The Internet brings forth many benefits, one of which is for network users to be socially connected with friends, families, and colleagues easily and conveniently, regardless of their actual geographical locations. There are many online social networks or forums, some of which, such as Facebook (http://www.facebook.com), MySpace (http://www.myspace.com), Flickr (http://www.flickr.com), and LinkedIn (http://www.linkedin.com), enjoy great success. These online social networks attract a great number of participants (i.e., social-network users) that contribute various types of content on the network. For example, social-network users may post messages, images, videos, audios, etc. on the network so that they may be shared among the social-network users or with the public in general. In general, a social-network user's activities in an online social network may include authoring, posting, or viewing network content and communicating (i.e., networking) with other social-network users.
Online social networks may be classified into two categories according to their different purposes. The first category includes networking oriented online social networks, such as Facebook and LinkedIn. These online social networks usually emphasize more on the social networking aspect of the networks, and the social relationship may be considered as the basis of such online social networks. With networking oriented online social networks, content sharing is mainly among friends. The second category includes knowledge-sharing oriented online social networks, such as blog networks, question-answer networks, and viral video networks. These online social networks usually emphasize more on the sharing of knowledge or network content. With knowledge-sharing oriented online social networks, the networks often are not driven by the underlying social relationships; instead, the networks are formed through their users' common interests in the shared content.